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Ecological Aspects of the Reproductive Biology of Eastern Kingbirds: Geographic Comparisons
Author(s) -
Murphy Michael T.
Publication year - 1983
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.2307/1937213
Subject(s) - biology , avian clutch size , incubation , ecology , incubation period , reproduction , zoology , geographic variation , population , demography , biochemistry , sociology
A geographic comparison of the ecology of Eastern Kingbirds (Tyrannus tyrannus) breeding in New York (NY), and Kansas (KS), USA, was conducted in 1979 and 1980, respectively Emphasis was placed on relating variation in reproductive parameters, both within and between sites, to season and weather. Superimposed on the geographic comparison was a severe drought in KS in 1980. In both populations egg laying peaked at the end of the 1st wk of June, and modal clutch size was three. seasonal declines in clutch size were also nearly identical. Clutch size differences were not significant. Eggs were significantly larger in NY than in KS. The difference was not related to either adult size or annual variation in weather. Egg mass was only weakly correlated with environmental variables (date and temperature), but interruptions in laying followed days of below—average temperatures. Egg mass increased With the order of laying in NY, but decreased in KS. This apparently reflected the stressful conditions for laying females in KS. Incubation was significantly shorter in KS, probably due to warmer air temperatures since an inverse relationship was found between air temperature and incubation length in NY. Smaller egg size in KS may also have contributed to the shorter incubation period. Nestling growth was relatively slow, and despite large mass differences (eggs were heavier in NY) growth rates were nearly identical. Absolute tarsus growth was also significantly greater in NY, but development of the ninth primary was significantly greater in KS. Nestling period length was inversely correlated with primary length in both populations. Emphasis on primary development in KS, where nest success was lower, vs. mass gain in NY, where starvation was more frequent, discussed in terms of tradeoffs to balance losses to predators (KS) and starvation (NY). Growth was negatively correlated with brood size in both populations, but in KS also with date, i.e., seasonal changes probably related to the drought. Productivity was greater in NY, but in neither population did it exceed 1.5 fledglings per nesting attempt. Mortality of fledglings is near zero in both populations due to intense postfledging parental care. Weather has important influences on the breeding biology of kingbirds, yet geographic variation in most traits was small. Hence, kingbird reproduction is not highly adapted to differences in average weather conditions between the sites. Occasional periods of severe weather probably preclude narrow specialization to local conditions. Kingbirds are therefore best characterized as a bet—hedging species. Patterns of growth and reproduction match hypothetical and independently derived predictions based on other aerial insectivores, thus providing support for several recent predictions for life history evolution.